Purpose

The grasslands in temperate east Asia share many features with comparable grasslands in
North America, including the strongly continental climate characterized by rainfall which
occurs during the warmer months of June, July and August and monthly temperatures which
range from 100 to 500 mm during these months. The grasslands of Asia and North America
(GANA) share some floristic similarities, often at the genera level. These grasslands have
evolved under a long evolutionary history dominated by grazing.

The grasslands of Inner Mongolia, China, have undergone changes in the extent of grazing
systems due to increased cropland conversion and intensification. The Mongolian portion of
the region has undergone less intensive changes in land use, and is noted for its greater
reliance on indigenous pastoral systems, including continual seasonal nomadic pastoral
patterns.

The purpose of this study was to examine the effect of climate and grazing on
distributions of photosynthetic types, and to test whether the SOM isotopic signal
reflects current vegetation inputs or whether plant community shifts have taken place.
This paper presents information on soil organic matter (SOM) C and isotopic composition at
several collection sites in the Mongolian steppe region.

Methods

Our research to compare ecosystem dynamics in grasslands of Asia and North America relies
upon a detailed floristic analysis of distribution, the use of stable isotopes to quantify
productive patterns and soil organic matter dynamics, and remote sensing to assess
geospatially explicit land cover performance. Research sample sites and nearby cities are
shown on seasonal land cover map of grasslands in the Temperate East Asia (Figure 1). The isotopic ratios not only
characterize carbon from C3 and C4 sources, it is fractionated during transfer through trophic
levels and is recorded in the SOM carbon, thereby allowing us to re-establish past carbon
sources into the SOM pool resulting from community shifts between C3 and C4 plants.

Carbon isotopic analysis followed procedures in standard use. A soil subsample was
de-carbonated with 0.5 N HCl and continuous stirring until no effervescence under vacuum
was detected. The treated sample was centrifuged, re-suspended in distilled water, and
re-centrifuged before the pellet was dried, pulverized and loaded into tin cups for
combustion under pure oxygen and at high temperatures in the Carlo Erba CHN analyzer.
Sample sizes were of variable size to provide adequate carbon for isotopic analysis. The
combusted sample was separated into CO2 and N2 gases and quantified with a gas chromatograph.

The combusted products were passed through a drying column, and CO2 was trapped at liquid
nitrogen temperature in the triple trap of a VG SIRA-10 Isotope Ratio Mass Spectrometer
(IRMS). Remaining gases were removed under high vacuum, and the CO2 was analyzed by the
IRMS.

Results

The typical grassland C3 values for the
Mongolian Steppe are clustered around -26.5% and C4 around -12.5% (Table
1). Soil carbon isotope values in the Mongolian Steppe ranged from -25 to -20 which
indicate a greater C3 input compared to estimates based on the North America regression (-21 to -17) (Figure 2). The Mandolgovi, Mongolia and
Donsheng sites (Figure 3a and d) indicate
a greater C4
contribution, perhaps due to the impact of grazing on plant community characteristics. The
soil carbon levels of these two sites also reflect greater grazing removal of plant
material, and both had low percent carbon values (approximately 1.5%). The Stipa
grasslands tend to be more prevalent in Mongolia, and the values of d13C and soil carbon
levels (Figure 3b and f) are typical of
well-managed Stipa grasslands of this region. In the more mesic portions of the Mongolian
Steppe, Leymus grasslands become an important community. The Leymus grasslands sampled at
the Inner Mongolian Grassland Ecosystem Research Station (IMGERS) at Xilingole, China are
indicative of d13C and soil carbon levels for this community (Figure 3c).

Analysis of climate relationship to d13C
patterns in the Mongolian Steppe indicate that July wind speed (positive), June
precipitation (negative), and June-July ratio of potential evapo-transpiration to
precipitation (positive) were the best predictors for soil carbon isotopes. All these
factors support the assertion that C4 plant species respond more favorably to water stress conditions
than C3
species. Soil carbon levels were sensitive to the same factors as the d13C isotopic
content, but responded in an opposite fashion. These relationships for soil isotopic
composition (d13C) and soil C based on climate data only are:

Soil carbon levels better correlated with growing season wind speed and precipitation in
grasslands of Mongolia, but annual mean temperature and growing season precipitation
better explained soil carbon content for grasslands in Inner Mongolia. The soil carbon
content decreased with precipitation in Inner Mongolia which appears to be related to land
use intensity.

Change of land use impacted ecosystem structure and functions in a number of ways,
altering plant composition, soil carbon, and soil isotopes. Relative C4 plant cover increases
with grazing relative to control in all research sites (Figure 4). Soil carbon at the depth 1-6 cm
was more sensitive to grazing management, increasing by about 10% with light grazing, and
decreasing by 25% with heavy grazing.

Conclusions

The predominant plant community of the Mongolian Steppe is
characterized by C3, cool season community.

Soil d13C values and soil organic matter are predicted by summer
precipitation and wind speed. Relationships derived from North American analysis
over-predicts the amount of warm season C4 plant inputs into these ecosystems.

Warm season communities do exist in the Mongolian Steppe in the
more arid regions of the steppe and in areas more heavily grazed.

Acknowledgments

Research was funded by an NSF-Long-Term Studies Program
and the NASA-EOS Program. Chemical analysis was completed with technical support of the
Department of Biology at Augustana College, Sioux Falls, SD, USA. Field support and
collaboration were provided by the Chinese Academy of Sciences and the Mongolian Academy
of Sciences.